Is Vertical Stiffness Associated with Common Traumatic Musculoskeletal Injuries in the Football Codes? An Observational Study with a Focus on Anterior Cruciate Ligament Injury

Injury to the anterior cruciate ligament (ACL) is a career limiting event for many sports people. Irrespective of a person’s activity level and whether they undergo surgery or conservative rehabilitation, ACL injury can lead to osteoarthritis later in life and other comorbidities. ACL injury rates remain high despite considerable research to reduce its incidence. Therefore, novel methods for preventing ACL injury are needed. In a published literature review presented in the introduction of this thesis it is noted that ACL injury incidence is greater in athletic populations, particularly in field and court sports, with approximately 50-80% of those being non-contact in nature. It showed little disagreement exists about the importance of dynamic knee joint stability for reducing ACL injury risk. To date, little research into dynamic knee joint stability has been conducted. This could be because of difficulty in measuring knee joint stability dynamically in-vivo, or due to difficulty identifying novel practices/methods which incorporate dynamic knee joint stability. This work proposed musculotendinous stiffness, or ‘stiffness’ as it is referred to in this thesis, as novel, relevant and worthy of investigation. A second literature review defined stiffness as a quantification of resistance against force. Specifically, vertical stiffness was described as the body’s resistance to vertical displacement from ground reaction force, affected by the interaction of connective tissue, muscle and bone, as well as stiffness at each joint. Therefore, this thesis addresses the question “is vertical stiffness associated with common traumatic musculoskeletal injuries in the football codes?”. A novel method of measuring dynamic knee joint stability and a surrogate measure of ACL loading in-vivo was applied which involved image registration of computed tomography with fluoroscopy to build a 4-D model of knee joint motion. This thesis is a compilation of published research papers. The study in chapter two showed hamstring and quadriceps pre-activation and co-activation is related to vertical stiffness for a task which simulates the manoeuvre typically observed when non-contact ACL injury occurs in field and court sports. Chapter three found no difference in vertical stiffness between a cohort of Australian Rules footballers who sustained a muscle strain injury and an uninjured cohort. This outcome was important because muscle inhibition can remain for 12 months following injury and, as identified in the previous study, thigh muscle function is important for vertical stiffness and, likely, knee joint stability. Chapter four used the novel technique described for measuring dynamic knee joint stability and ACL elongation in-vivo on a step-up task while also measuring hamstring and quadriceps activity. It argued hamstring and quadriceps co-activation is not associated with ACL elongation. Finally, chapter five used the same method to measure dynamic knee joint stability on a task similar to that used in chapter two and failed to find a link between vertical stiffness and ACL elongation. This work found no evidence of an association between vertical stiffness and some common traumatic non-contact injuries. It concluded training programs which enhance vertical stiffness may be implemented without concern for injury. Suggestions for future research are also made